This invention relates to transistor amplifier stages with gain control circuits, and more particularly to such circuits which cause the decibel value of the net gain of the stage to vary in discrete steps of substantially the same magnitude.
Line amplifiers in telecommunication systems typically have some form of manual gain adjustment for setting levels throughout a system. Gain changes are normally provided in discrete logarithmic (decibel) steps of the same value for convenience. The prior-art line amplifier disclosed in U.S. Pat. No. 3,778,563 uses a resistive L-pad on the input of a fixed gain amplifier. Each section of this L-pad comprises a series resistor between an input line and the input of the amplifier, and a shunt resistor connected to one end of an associated series resistor. A plurality of such sections may be connected in series without substantially changing the input impedance of the amplifier. The decibel values of attenuation per section of the pad therefore add up on a linear basis. The attenuation of the input pad is decreased (i.e., the net gain is increased) by removing sections therefrom. This is accomplished by closing one shunt screw switch to short-circuit the associated series resistor and at the same time opening a series screw switch to open-circuit the associated shunt resistor. Thus, it is seen that at least two screw adjustments are required there for each step change in attenuation or gain. Since the overall gain of this prior-art circuit is adjusted by removing resistive elements from the input circuit thereof, the amplifier following the pad is normally adjusted to have a relatively high gain. Such a high-gain amplifier is inherently more susceptible to noise, oscillation, and increased power drain than an amplifier normally operating at a lower level of gain.
An object of this invention is the provision of an improved variable gain amplifier stage. Another object is the provision of an improved gain control circuit for an amplifier stage in which gain changes provided by individual switch means are cumulative. Another object is the provision of an improved resistive pad.